Dragline excavating systems have long been used in mining and earth moving operations. Unlike other excavating machines, dragline buckets are controlled and supported solely by cables and chains. To a large extent, the stability and performance of the bucket in operation must come from the construction of the bucket.
In smaller buckets, the forces encountered in a dragline operation are not great and the payloads are small. With these buckets, the forces and payloads are easy to compensate for without inhibiting the operation. Even if a small bucket possess an inefficient design, the difference in fill times is not great because the bucket capacities are small. However, with the increasing size of machines, mines and desire for greater production, dragline operations have grown considerably in size over time. In today's mines, large dragline buckets on the order of 30 cubic yards and larger are common, and buckets up to 175 cubic yards are in use. In large buckets, the design paradigm changes because the shear forces of the material to be excavated (e.g., the ground), which substantially impact the design of smaller buckets, become less important in comparison to the large loads imposed on large buckets. The expanse and massiveness of these buckets, the large size of the payloads, and the very high forces applied by the drag chains during a digging cycle require different considerations. Yet, many bucket designs still follow old or imperfect rules that fail to optimize the bucket digging performance. As a result, many problems still exist in today's dragline buckets.
Since there is no stick or hydraulic cylinder to power the bucket into the ground, it is important for the bucket to be able to dig into and penetrate the ground when the drag ropes pull the bucket toward the prime mover. To maximize production, it is desirable for the bucket to penetrate into the ground as quickly as possible. Many older buckets were constructed with a heavy front end to withstand the rigors of mining. Such an arrangement placed the center of gravity at a relatively high and forward portion, which caused the bucket to tip forward onto the teeth when pulled forward. The operator needed to exercise great care with these buckets to avoid tipping the bucket too far forward and over on its front end. Even if the bucket is kept in a digging position, it still tends to remain tilted too far forward such that the material is subject to substantial disruption during loading. Moreover, primarily due to roll piles, great force is required to pull such a tilted bucket through the ground. On the other hand, buckets with the center of gravity shifted further toward the rear wall tend to penetrate more gradually and with more difficulty, which leads to longer fill times and diminished productivity. U.S. Pat. No. 4,791,738 to Briscoe discloses an increasing pull to tip concept that alleviates the risk of tipping the bucket over while still facilitating better and surer penetration into the ground. While this design concept improves dragline operation, the buckets still experience a relatively gradual and shallow penetration that requires increased translation of the bucket for filling. FIG. 7 illustrates a generalized penetration profile P1 of ground G for one example of a conventional bucket.
Dragline buckets are provided with a bottom wall, a pair of opposite sidewalls upstanding from the bottom wall, and a rear wall at the trailing end of the sidewalls. The walls collectively define an open front end and a bucket cavity to collect the earthen material. A lip with excavating teeth and shrouds extends across the front end of the bottom wall to enhance penetration and digging, and reduce wear of bucket structure. The sidewalls generally taper from top to bottom and from front to back to ease and speed dumping of the gathered material. Incomplete dumping in dragline buckets leads to material being carried back for the next digging stroke. This problem not only requires unnecessary weight being hauled around, but also diminishes the production of each digging stroke, i.e., less new material can be gathered because old material remains in the bucket.
In a conventional bucket, the mass of earthen material being gathered is forced generally inward and upward by the tapered sidewalls through about one half to two-thirds of its travel through the bucket toward the rear wall, where it thereafter tends to fall toward the bottom and rear walls. This piling of the material causes it to build up in a heap toward the front of the bucket. The formation of such a heap within the bucket requires increased force on the drag ropes, slower filling, and a build up of the material in the front of the bucket. Once the heap reaches a certain mass it begins to act almost like a bulldozer blade plowing the material forward in front of the bucket. Such heaps also commonly cause roll piles to be formed in front of the buckets (i.e., dirt that heaps up and rolls forward in front of the dragline buckets). In some operations, roll piles need to be periodically smoothed: by other equipment (such as by bulldozers) to avoid obstruction and wearing of the drag ropes. In other operations, bulldozers or other equipment are used push roll piles away from the prime mover in order to provide adequate resistance in a digging operation at a position far enough away from the prime mover to permit the bucket to fully load before it reaches the end of its translation in a digging stroke. That is, the roll piles are sometimes used to load the bucket during subsequent passes and are often necessary to fill the bucket.
To provide large payloads and withstand the extreme loading and stresses in modern dragline operations, the buckets themselves are ordinarily massive structures. To reduce wearing, the buckets are typically provided with a wide variety of wear parts which further increase the weight of the bucket. The rigging to accommodate and control such large buckets is also of substantial mass and weight. The boom and prime mover are designed to accommodate a maximum load, which is a combination of the weight of the dragline bucket, the wear parts, the rigging, and the excavation material within the bucket. The greater the weight of the rigging and the dragline bucket, the lesser the capacity remaining available for loading earthen material within the dragline bucket. While some efforts have been made to reduce rigging weight, it has largely resulted in only small incremental reductions or led to other undesirable problems.
Further, the bucket and rigging components are exposed to a highly abrasive environment where dirt, rocks, and other debris abrade the rigging and the dragline bucket as they contact the ground. Connections between rigging elements also experience wear in areas where they bear against each other and are subjected to various forces. Following a period of use, therefore, the dragline excavating system must be subjected to periodic maintenance so that various parts can be inspected, replaced or repaired. In most modern systems, there are many parts that require such inspection, repair or replacement and it takes significant downtime of the operation to complete the needed tasks. Such downtime decreases the production and efficiency of the dragline operation.